In recent years, as a radical solution of energetic and/or environmental problems, and, further, as a central energy conversion system in the future age of hydrogen energy, fuel cell technique has drawn attention. Especially, polymer electrolyte fuel cells (PEFC or PEMFC) are tried to be applied as power sources for electric automobiles, power sources for portable instruments, and, further, applied to domestically stationary power source apparatuses utilizing electricity and heat at the same time, from the viewpoint of miniaturization and lightening of cells, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of a polymer electrolyte membrane having proton conductivity, catalyst layers comprising a platinum group metal catalyst supported on carbon powder and an ion-conducting binder comprising a polymer electrolyte are formed, respectively. On the outsides of the catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are bonded to the electrolyte membrane so that the catalyst layers can face to the electrolyte membrane, respectively, is called a membrane-electrode assembly (MEA). On both sides of the membrane-electrode assembly, separators having electric conductivity and gastightness are placed. Gas paths supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane-electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and supplying an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
As polymer electrolyte membranes for polymer electrolyte fuel cells, Nafion (registered trade mark of Dupont Co., as is the same hereinafter), which is a perfluorocarbonsulfonic acid polymer, is generally used by reason of being chemically stable. However, Nafion is very expensive because of a fluoropolymer. Moreover, Nafion has a problem that, when methanol is used as a fuel, a phenomenon that methanol permeates the electrolyte membrane from one electrode side to the other electrode side (methanol crossover) is liable to occur. Furthermore, fluorine-containing polymers contain fluorine, and consideration to the environment at the time of its synthesis and disposal becomes necessary. From these backgrounds, development of novel polymer electrolyte membranes is desired.
An example of study of ion-conducting polymers using a polyvinyl alcohol as a base, as ion-conducting polymers using a non-fluoropolymer as a base is known. Polyvinyl alcoholic polymers are inexpensive resins and easy to mold into film, and, therefore, considered to be useful as ion-conducting polymer membranes of low costs. Furthermore, since polyvinyl alcoholic polymers do not substantially contain halogens, they have an advantage of a small load on environment at the time of scrapping. For example, there is a report of an example using a polyvinyl alcoholic polymer having sulfonic acid groups obtained by copolymerizing a monomer having a sulfonic acid group as an ion-conducting group with vinyl acetate and then saponifying the resulting copolymer (Non-patent Document 1). However, since vinyl acetate as a starting material of polyvinyl alcoholic polymers is generally poor in copolymerization reactivity with other copolymerizable monomers, introduction of a large amount of ion-conducting groups into the polymer is difficult, and. as a result, display of ion conductivity, which is important as an ion-conducting polymer, is difficult. For solution of this problem, there is an example wherein a blend of sulfonic acid groups-containing polystyrene and a polyvinyl alcohol (Patent Document 1) or a substance obtained by cross-linking of the blend (Patent Document 2) is studied, but, in the first place, polyvinyl alcoholic polymers are poor in compatibility with polystyrene-type polymers, and they furiously phase separate from each other, and, therefore, it is presumed that there is a problem in homogeneity of the membrane. Separately, there is a report on a method of generating ion conductivity by mixing an inorganic acid with a polyvinyl alcoholic polymer (Patent Document 3), but, when water is used or when an aqueous methanol solution is used as a fuel (direct methanol fuel cell), inorganic salts contained in the electrolyte membrane are eluted into the aqueous methanol solution. As a result, the inorganic salts move into the catalyst layer side, and the concentration of the inorganic salts in the electrolyte is lowered and the ion conductivity of the membrane is lowered.
Thus, it is the actual situation that no polyvinyl alcoholic polymer electrolyte membrane which is economical and has improved performance has been proposed.    Patent Document 1: JP 5-174856 A    Patent Document 2: JP 6-76838 A    Patent Document 3: JP 2004-146208 A    Non-patent Document 1: Kobunshi Gakkai Yokoshu (Polymer Society Abstracts) 54 (1), page 1755 (2005)